1. Field of the Invention
This invention relates to a process of producing cement clinker comprising preheating the raw cement powder in a suspension-type heat exchanger, de-acidifying in a precalcining system, clinkering in a fluidized bed, and subsequently cooling the clinker.
2. Discussion of Prior Art
In the production of cement clinker it is generally usual to remove mechanically and chemically bound water from raw cement powder, to de-acidify the raw cement powder to a certain degree in a suspension-type heat exchanger and to use a rotary kiln for the further de-acidification and the clinkering.
Because a major part of the de-acidification is effected in the rotary kiln in that practice, it has the disadvantage that particularly for high throughput rates the rotary kiln must have large dimensions, which are not desirable from the aspect of plant engineering.
For this reason numerous efforts have been made to deacidify the raw cement powder as far as possible before it enters the rotary kiln. For this purpose, e.g., fuel has been fed separately to the lowermost stage of the suspension-type heat exchanger in order to effect a further heating a de-acidification of the raw material (German Patent Publication 23 24 565). But the effect of that practice is limited because the raw material is subjected to the elevated temperature only for a short time.
A further development for in the production of cement clinker calls for de-acidifying the raw material as far as possible in a separate unit, which is connected between the suspension-type heat exchanger and the rotary kiln and may consist of a heated pneumatic conveyor for feeding the raw material from the suspension-type heat exchanger to a cyclone, which precedes the rotary kiln (French Pat. No. 2,197,827), or of a fluidized bed reactor, from which the raw material is charged into the rotary kiln through a smoke chamber and a cyclone (German Pat. No. 23 44 094). In both cases it is difficult to control the residence time which is required for an adequate de-acidification, particularly because the grading of the raw material usually varies. It is also difficult to discharge the coarse particles.
In a process of burning raw cement powder in a plant comprising a suspension-type heat exchanger and a precalcining unit it is also known to burn the material to form clinker in a fluidized bed (European Opened Application 00 00 739). That practice has the disadvantage that it is virtually impossible to maintain in the clinkering stage the uniform residence time which is essential for a cement of high quality.
It is an object of the invention to provide a process in which the disadvantages of the known processes, particularly of those described hereinbefore are avoided, which permits a virtually complete de-acidification of the raw cement powder before it is charged to the fluidized bed furnace and which can be carried out with a reasonable structural expenditure.